1. Technical Field
The present invention relates generally to methods of fabricating a semiconductor device, and more particularly, to methods of fabricating a semiconductor device using a dilute aqueous solution of an ammonia and peroxide mixture (hereinafter abbreviated as “APM”).
2. Discussion of the Related Art
Recently, semiconductor fabrication processes have focused on forming a transistor on a silicon-germanium layer (rather than on a substantially homogeneous single crystalline silicon substrate) in order to meet growing market demand for semiconductor devices having properties of higher speed and a higher degree of integration. The silicon-germanium layer is typically formed by appling germanium atoms to a surface of a single crystalline silicon substrate to form a silicon-germanium alloy thereon.
Using a silicon-germanium (SiGe) layer in semiconductor fabrication increases the mobility of a carrier by using a lattice constant different from the single crystalline silicon substrate conventionally used in semiconductor devices. The increase in the mobility of the carrier results in improving a current driving capability of the transistor. This gives a breakthrough for relatively easily improving the speed of a transistor of any given size rather than improving the speed by arbitrarily reducing a size thereof. The lattice constant of the silicon-germanium (SiGe) layer has been found to be proportional to the amount of germanium atoms contained in the SiGe layer.
However, the silicon-germanium layer has the disadvantageous property of being vulnerable to an aqueous solution containing ammonium hydroxide and hydrogen peroxide. This is because the silicon-germanium layer readily reacts with hydrogen peroxide (H2O2) to first form a silicon oxide layer, and that silicon oxide layer is then excessively etched by the ammonium hydroxide. Such an excessively etched silicon-germanium layer significantly impairs the performance characteristics of the resulting transistor.
U.S. Pat. No. 6,399,487 to Jane-Bai Lai, et. al (the '487 patent), which is incorporated herein by reference, discloses a method of reducing phase transition temperature by using silicon-germanium alloys.
According to the '487 patent, the method includes forming a gate of a polysilicon layer on a silicon substrate and forming a silicon-germanium alloy layer on the gate. Thereafter, a titanium layer is formed on the silicon-germanium alloy layer through a salicide process, along with forming source and drain regions overlapping the gate. The '487 patent further teaches that the titanium forms a titanium alloy wherever the titanium is in contact with the silicon-germanium alloy, but that unreacted titanium may subsequently be removed from the silicon substrate by an etching step using an aqueous solution of ammonia and hydrogen peroxide “which results in removal of all unreacted titanium, leaving behind conductive material over the source, drain, and gate but not over the spacers.”
However, the aqueous solution of ammonia and hydrogen peroxide used in this step may excessively etch the upper surface and the sidewalls of the silicon-germanium alloy unless the volume ratios of ammonia and hydrogen peroxide in the aqueous solution are optimized and contact time is carefully controlled.
These and other problems with and limitations of the prior art approaches to fabricating semiconductor devices which include a silicon-germanium layer are addressed in whole or at least in part by the methods of this invention.